The switch could put the Missouri case on a more direct route through the federal appellate courts at a time when U.S. judges have been increasingly striking down other states' gay-marriage bans.

"We wanted at least one of the cases (from Missouri) to be considered in a court of broader jurisdiction," Koster spokesman Eric Slusher said Thursday.

At issue is a lawsuit attorneys for the American Civil Liberties Union filed earlier this summer in Jackson County Circuit Court on behalf of two same-sex couples who were denied marriage licenses.

Although the state was not named as a defendant, Koster's office intervened because the lawsuit alleges that Missouri's constitutional prohibition on same-sex marriage violates equal-protection and due-process rights under the U.S. Constitution.

Because of the alleged federal violations, Koster's office moved the case to U.S. District Court, where it has been assigned to Judge Ortrie Smith.

Koster, a Democrat who is running for governor in 2016, has said he personally supports gay marriage but will carry out his official duties by defending Missouri's ban. His office filed federal court documents earlier this week arguing that Missouri's prohibition should be upheld because states have a right to set their own rational constraints on domestic relations until the U.S. Supreme Court says otherwise.

Missouri voters adopted a state constitutional amendment in 2004 that limits marriage to one man and one woman. The Legislature passed a similar state law in 1996.

The lawsuit contends the prohibition furthers no legitimate government interest and "serves only to disparage and injure same-sex couples."

ACLU attorney Tony Rothert said the switch to federal court means there is "one less step to get to the U.S. Supreme Court." But he said that may not matter.

"We're very much at square one," Rothert said. "It's unlikely that this particular marriage-license case will be decided before the U.S. Supreme Court tells us the answer" to similar challenges raised in other states.

The decision to move the case to federal court does not affect a separate legal battle in St. Louis Circuit Court that arose in June after the city issued marriage licenses to several gay couples despite the Missouri prohibition.

Challenges to state gay-marriage bans have gained momentum after the U.S. Supreme Court last year ordered the federal government to recognize state-sanctioned gay marriages. Partly as a result of lower court rulings, gay couples now can marry in 19 states and the District of Columbia.

Earlier this week, officials from Utah and Oklahoma asked the U.S. Supreme Court to hear appeals of decisions striking down their states' gay marriage bans.